Origin of the term Red-Light District

The “red-light district,” or the place in a city where commercial sex is isolated or encouraged (or both), might be a concept now most associated with Europe and Asia, but it's an American invention. The Oxford English Dictionary puts the first print appearance of the phrase at 1894, in the Ohio newspaper the Sandusky Register, in reference to a group of Salvation Army volunteers who had set up shop in town to minister to presumed prostitutes. The term has its origins in the practice not of prostitutes, but their customers: in this case, rail workers who left red lanterns outside the doors and windows of the houses where they met prostitutes between their own work shifts. If their boss needed to find them, he could look for the light.

From  Alternet: When Prostitution Wasn't a Crime: The Fascinating History of Sex Work in America